This application relates to a continuous process for producing oligocarbonates by the melt transesterification process.
A continuous process for producing oligocarbonate is disclosed. The process entails in sequence the steps of a) reacting a dihydroxyaryl compound with a diaryl carbonate in the presence of a catalyst to obtain a melt and avoiding removal of released monohydroxyaryl compound and b) withdrawing the monohydroxyaryl compound by flash evaporation and c) heating the melt obtained under vacuum in an evaporator equipped with sump receiver and d) repeating at least once steps b) and then c) each at gradually increased temperatures and lowered pressures.
Oligocarbonates constitute the first step in the production of polycarbonates and are obtainable from dihydroxyaryl compounds and diaryl carbonates in the presence of one or more catalysts. The oligocarbonates have weight-average molecular weights (Mw) of 3,000 to 20,000, preferably 3,000 to 15,000 and are condensed to form polycondensates having molecular weights Mw of 15,000 to 40,000.
The production of oligocarbonates by the melt transesterification process is known in principle. A discontinuous process is described, for example, in DE-A 4 312 390. Further details of the melt transesterification process are described in the literature (see, for example, Hermann Schnell, Chemistry and Physics of Polycarbonates, Polymer Reviews, Vol. 9, 1964, pages 44 to 51, DE-A 1 031 512, U.S. Pat. No. 3,022,272, U.S. Pat. No. 5,340,905 and U.S. Pat. No. 5,399,659).
Regarding continuous techniques, several reaction vessels are generally used for the formation of oligocarbonates; these reaction vessels are operated batchwise in alternation, in order subsequently to obtain a continuous flow of product. This process is described, for example, in U.S. Pat. No. 5,912,318, U.S. Pat. No. 5,932,683, U.S. Pat. No. 5,912,289, WO 00/26 276 or EP-A 620 240. This procedure has the disadvantage, however, that the batch-operated vessels have long residence times of 1 to 20 hours and lead to oligomers of varying quality which have an adverse effect during the polycondensation. In addition, the temperature or the vacuum in the vessels, such as, for example, in EP-A 620 240, is slowly increased with each batch in order to remove released monohydroxyaryl compound. This requires additional time and energy, as the vessel has to be re-cooled for the next batch. Moreover, a scale-up of the agitated tanks rapidly comes up against limiting factors, because the available mass-transfer surface area in proportion to the volume steadily becomes less.
Consequently, the object of the present invention was to find a process for producing oligocarbonates which can be operated continuously and renders possible a better heat input and shorter residence times in the reaction chamber.